Image orthicon glass target with aluminum-tantalum oxide coating



4. NEY7 April 8, 1969 IMAGE ORTHICON GLASS TARGET wrra ALUMINUM-TANTALUM OXIDE comma rile April-.6, 1967 FIGJ INVENTOR ROBERT J. NEY,

IS ATTORNEY.

United States Patent Ofice 3,437,860; Patented Apr. 8, 1969 3,437,860 IMAGE ORTHICON GLASS TARGET WITH ALU- MINUM-TANTALUM OXIDE COATING Robert J. Ney, Liverpool, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 6, 1967, Ser. No. 628,965 Int. Cl. H01j 31/26; Hk 3/00; H05b 33/28 U.S. Cl. 313-65 2 Claims ABSTRACT OF THE DISCLOSURE A storage target for image orthicons or other camera tubes having an improved protective coating thereon, said protective coating consisting of an oxide of an alloy containing 2-10 percent tantalum and the balance aluminum.

The present invention relates to improvement in image orthicons and particularly to an improved electric charge storage target for producing a point-by-point electric charge pattern corresponding to a visual image or other information to be converted to electrical signals by scanning the target electrode wit-h an electron beam. More particularly the invention relates to a storage target having an improved protective coating thereon of the type disclosed in copending application Ser. No. 528,926, filed Feb. 21, 1966, Donald L. Schaefer, and assigned to the same assignee as herein.

As is well known to those skilled in the art, in the operation of a camera tube of the image orthicon type light falling on the photocathode of the tube produces emission of electrons which are accelerated axially toward and impinge upon a transversely extending charge storage target of thin sheet or membrane-like form. The primary electrons arriving at the charge storage target produce emission of secondary electrons which are collected by a closely spaced confronting foraminate or mesh-like electron collector electrode so as to leave a charge pattern on the storage target which is representative of the optical image supplied to the photocathode of the tube. The stored information is read out as modulation of the return current of an electron beam which scans the charge storage target and neutralizes the charge thereon. To provide charge neutralization by the target scanning electron beam suflicient to avoid after-images or stickiness of the target, the time constant of the charge transfer mechanism must be compatible with the target scanning rate, and hence the resistivity of the target in the transverse dimension or neutralization current direction thereof is an important target characteristic, and must be within desired limits, such limits being in the range 1 10 to 1x10 ohm centimeters for television scan applications. Another important characteristic of the target, which in large measure determines the sensitivity of such a tube, is the secondary emission ratio of the target responsive to arrival of the primary electrons from the photocathode. Hence a target material of high secondary electron emissivity is desirable for high sensitivity. The signal to noise ratio of the tube is also importantly affected by the spacing of the target and secondary electron collector electrode, a very close spacing of the two, such as 0.5 to 2 mils, being desirable for acceptably high signal to noise ratios. Thus mechanical strength and rigidity of the target, and resistance to vibrational movement such as to change or vary the target-to-collector electrode spacing during tube operation, is also an important consideration in respect to such targets. A further problem associated with image orthicon targets and particularly electronically conducting glass targets, is cesium attack from cesium vapor in the tube as a result of photocathode treatment with cesium. Cesium, or other alkali metal attack results in a drop in resistivity of the target glass. These problems are largely overcome by virtue of the inventions and discoveries disclosed and claimed in the aforementioned copending application Ser. No. 528,926. However, greatly improved results may be obtained in accordance with the present invention, as shall be pointed out more particularly hereinafter. It is a principal object of this invention to provide an improved protective coating for electronically conductive glass targets.

Thus, the preferred form of this invention relates to the provision of a homogeneous polycrystalline semiconducting metal oxide protective layer consisting of the oxide of an alloy containing 2-10 percent tantalum and the balance aluminum. An electronically conducting glass is utilized as a camera tube target. Targets made in accordance with the invention exhibit no measurable cesium attack, improved secondary emission characteristics, uniform coating characteristics, and low noise. Most surprisingly, tubes having such targets have exhibited improved outputcharacteristics with use over a period in excess of 8000 hours.

Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below taken in conjunction with the drawings accompanying and forming a part of this specification, in which:

FIGURE 1 is a view, partially broken away in axial section, of the image section of an image orthicon having a charge storage target constructed in accordance with the present invention;

FIGURE 2 is an enlarged sectional view of a portion of the target and its support; and

FIGURE 3 is an exploded view thereof.

In the following description, the term glass will be understood to relate to the electronically conducting borate, silicate, phosphate, and germanate glass as more completely detailed in copending application Ser. No. 528,926.

Referring to FIGURE 1, there is shown the image section of an image orthicon tube having an improved charge storage target constructed in accordance with the present invention. Within the envelope 2 of the tube and on the front face or viewing window 4 thereof there is disposed a suitable photocathode 6. Responsive to an optical image falling on the photocathode 6 electron-s are emitted and accelerated rearwardly and axially by the accelerating effect of internal electrodes 8, 10, and focused by the magnetic field of an external focusing magnet (not shown). The primary electrons from photocathode 6 fall on the forwardly facing surface of the transversely disposed membrane-like imperforate charge storage target 12. This in turn produces emission of secondary electrons from the target 12 which are collected by the adjacent mesh-like or foraminate electron collector electrode 14. This produces a pattern of charge stored on the forwardly facing surface 16 of the target 12, which by electric conduction through the target produces a corresponding pattern of charge on the rearwardly facing surface 18 of the target 12. The latter charge pattern is then read out by the neutralization action of a target scanning electron beam 20, generating a readout signal in the form of modulation of the current of the return portion 22 of the beam.

The improved target constructed in accordance with my invention consists of a base layer 30 (on FIGURE 2) of electronically conducting glass which is secured at its periphery to, and may be exclusively supported by, annular support or ring 32. The base layer 30 extends across support 32 in drum head fashion with suificient tautness desirably to avoid sags or wrinkles and to preclude excessive vibration or movement relative to mesh electrode 14 under conditions of tube vibration.

Directly superimposed on layer and overlying it in direct contact therewith on the side facing the photocathode is a second layer 36 consisting of homogeneous polycrystalline semiconducting metal oxide having a secondary emission ratio higher than that of the glass layer 30. The layer 36 is composed of the oxidation product of an alloy consisting essentially of 3% tantalum and the balance aluminum. The glass layer may have a thickness in the range of 0.5 mil to 0.3 mil, and the aluminum tantalum oxide layer 36 may have a thickness in the range of 100 to 1000 angstroms. The thickness of the oxide layer is quite critical since below the minimum range, the target is corrosively attached by cesium while above this range long afterimage is encounter since the increased electrical resistance effects are undesirably long discharge time constant.

In the manufacture of the composite target a bubble of the fresh glass of layer 30 is first blow, and a portion of sufficient size is cut out of the solidified bubble. The selected portion of the bubble is heated sufficiently so that it grows soft and sags or may be pressed fiat and secured to the annular support 32. A thin layer of an aluminum-tantalum alloy is then vapor-deposited on the glass 30 and heated in an oxidizing atmosphere, for example by baking in air at a temperature up to 375 C. for several hours, sufficiently to convert it to oxide 36. The glass 30 does not fill the interstices between the individual grains of the crystals of the oxide layer 36, and hence does not interfere with the desired electronic conduction substantially straight through the grain boundaries of the layer 36.

Targets made in accordance with the invention have surprisingly shown improved electrical characteristics with use. For example, new 3-inch tubes having these targets have shown a signal-to-noise level ratio of 72:1 while after several thousand hours of life the noise level ratio has improved to over 90:1.

The secondary emission coefficient of the tantalum aluminum oxide is about 4-7 and this compares favorably to 23 that of the prior art soda-lime glass targets. The borate target is quite resistant to atmospheric moisture attack or hazing, and the overlay is uniform and grain free. On a properly processed target, no glass surface texture can be detected at 500 magnification. This contributes to the low noise properties of this target.

In accordance with further aspects of the invention, the aforementioned target 12 is securely mounted in accelerator electrode 10 by the target support assembly 38, shown in exploded view in FIGURE 3. The target support assembly includes a front cover 40 and a rear cover 42 for the target 12 and collector electrode 14. Interposed between the target 12 and the collector electrode 14 is a spacer ring 44 and between cover 40 and collector electrode 14 is a wave spring washer 46.

The collector electrode 14, as shown in FIGURE 2, consists of a target mesh 48 which is stretched tightly over a mesh support ring 50 and secured thereto by mesh clamp ring 52.

The completed target assembly provides a secure, yet

resilient, mounting for the target 12 since the front cover 40 is maintained at a predetermined spacing from rear cover 42 by a peripheral flange 4011 which coacts with rear cover flange 42a. The covers 40 and 42 are secured together by a plurality of straps 54. Thus, the spring washer 46 exerts a force against the collector electrode 14, which, in turn, exerts a force on the target 12 through spacer ring 44. It will be noted that spacer ring 44 bears directly against the target membrane 30, 32, thereby improving the tautness of the target 12 in the area scanned by the electron beam.

In all respects, the disclosed tube is as good as the industry standard soda-lime glass target tubes, and in most respects it is far superior since resolutions at knee pluS one, in excess of 1400 TV lines, using a wideband camera chain, can be obtained. Signal-to-noise ratios as high as as 120:1 were observed on 3-inch orthicon tubes.

It will be noted that the invention is applicable to standard ionically conductive glasses of the prior art as well.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illustrative embodiments heretofore described. Accordingly, it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a target electrode for a camera tube or the like comprising an electronically-conducting glass selected from the group consisting of borate, silicate, germanate and phosphate glasses having a room-temperature electrical resistance of the order of 10 to 10 ohm-cm, and having a thin homogeneous polycrystalline semiconduct ing metal oxide layer thereon, and improved oxide layer composed of the oxidation product of an alloy consisting essentially of 2l0% tantalum and the balance aluminum.

2. A target electrode for a camera tube or the like as recited in claim 1, wherein said improved oxide layer is composed of the oxidation product of an alloy consisting essentially of 3% tantalum and the balance aluminum.

References Cited UNITED STATES PATENTS 2,743,150 4/1956 Rudy 313 X 3,004,875 10/1961 Lytle 117211 3,061,752 10/1962 Banks 31365 3,202,854 8/1965 Ochs 313-65 3,303,373 2/1967 Alting-Mees 31365 FOREIGN PATENTS 976,599 12/ 1954 Great Britain. 987,657 3/ 1965 Great Britain.

ROBERT SEGAL, Primary Examiner.

US. Cl. X.R. 117-211 

